Engineering Near‐Zero Thermal Expansion in Single‐Phase Oxides: A TTB‐Structure Platform Enabled by Entropy‐Driven Doping

ABSTRACT Achieving near‐zero thermal expansion (NZTE) in single‐phase materials over broad temperature ranges is crucial yet challenging for precision applications. Here, we report a new family of NZTE materials based on triple tetragonal‐tungsten‐bronze (TTB) oxides, including Ta 8 W 9 O 47 (TWO), (Nb 0.5 Ta 0.5 ) 8 W 9 O 47 (NTWO), and (Nb 0.5 Ta 0.5 ) 8 (Mo 0.5 W 0.5 ) 9 O 47 (NTMWO), synthesized via solid‐state reaction and entropy‐driven cation doping. High‐temperature X‐ray diffraction reveals anisotropic lattice behavior in all compositions: positive expansion along the a ‐ and b ‐axes counterbalanced by negative expansion along the c ‐axis. This compensation yields average linear coefficients of thermal expansion (CTE) near zero ( α l < 1.30 µK −1 ) from 300 to 1673 K. Notably, Nb‐doping in NTWO optimizes this balance, achieving the lowest α l of 0.656(43) µK −1 . Distinct temperature regimes highlight specific advantages: TWO maintains stable NZTE ( α dil = −0.455(1) µK −1 ) from 173–773 K; NTWO exhibits ultralow CTE (−0.079(1) µK −1 ) between 473–773 K; and NTMWO demonstrates exceptional dimensional stability at cryogenic temperatures (93–300 K, α dil = −0.096(1) µK −1 ). The bulk NZTE behavior stems from the synergy between the intrinsic anisotropic lattice expansion and microstructural features, such as rod‐like grains and porosity. These results establish Ta 8 W 9 O 47 ‐type TTB oxides as a robust, non‐ferroelectric platform for designing single‐phase NZTE ceramics for precision and aerospace applications.